300W should be fine with that hardware.

It should if it's good power supply. There was a test of PS units posted on ntcompatible a few weeks ago (It was on a german site). I do favor intel processors and curently I have a 1.3 G celeron, 2 maxtro 40 gb, 1cdrom, 1cdrw. And I'm sure I could hook up a lot more without getting out of power. You can go to the AMD site and look up the info how much watts your processor uses, I think HDD's use around 5watts (23 spinup peak). Cdroms should be something simmilar. I'm not sure how much the Motherboard uses, but it's not that much. You should also add your gpu wattage (If you have a nice 3d one) and I doubt that your computer will use more than 200w. Two od my friens have a simmilar configuration like you, the one has 1600xp and the other 1800xp. They both got 300w psu's and no power related problems. At a party once I ran my Celeron as an mp3 player with a 100w car amp connected to the computer psu, and the computer didn't crash (Maybe it would if I tried to play quake, but didn't try it). So I think you're good with 300w.

Definitely make sure it's a high-quality PSU. When I put together my P4 box, the 300W PSU didn't work. It was ATX 2.03 compliant, but it wasn't good enough to power up the system. It currently runs fine powering a dual P3 box with several drives---kinda wierd to me, but hey, that's just my experience. On the other hand, you can't have too large of one either. Again, you don't want a POS either here, but if it's not that much more go for a 350 or 400W---just to be safe. My P4 box is pretty basic in it's configuration 1 hdd, 1 optical, video card, NIC...not overclocked presently. YMMV, but just pick your PSU carefully like one from Antec, Sparkle or Enermax.